The generally accepted math, in polio detection, is that one verified case of polio paralysis represents up to 200 cases of silent infection. Those 200 undetected cases may not experience symptoms, but they can pass on the disease to others. As a result, one case of polio in an area that has been considered polio-free is an emergency. Four cases, as you can imagine, is much worse.

The WHO and Global Polio Eradication Initiative say that the virus in the four children has been sequenced, and is closely related to poliovirus type 1 that is currently circulating in Pakistan. Pakistan is one of the four countries where circulation of poliovirus has never been interrupted.

Here’s what the area looks like (map adapted from this one); the green area is Xinjiang province and the red is Hotan prefecture. (Yes, I admit it, I suck at Photoshop. Cut me some slack, this is breaking news.) The important point on this map is that three of the four countries where polio virus continues to circulate freely — Nigeria, Afghanistan, Pakistan and India — share borders with Xinjiang province. Two of them, Pakistan and India, border directly on Hotan. (Exactly where India’s border with Xinjiang falls is a matter of long-standing dispute.)

The conclusion is very clear — the indispensible listserv ProMED made it explicit — that this infection was carried over the border from Pakistan into China. ProMED suggests that polio may have been circulating in the area for at least two years before being detected via these new cases.

There are micro and macro meanings to this development, and none of them are good.

At the micro level: Of the four endemic countries, Pakistan is considered the most problematic, because it shares not only the biological barriers that make eradication difficult in neighboring India, but also profound political unrest. But Xinjiang is an area of political unrest too; it is 45 percent Uighur, the Muslim ethnic group who have been pressing for independence from China. In 2009, the province rioted and the riots were brutally suppressed; sporadic violence continues there, most recently a few weeks ago. The Chinese government considers the area “under terrorist threat” from the separatist movement, and last week announced a 2-month crackdown on political activity and also on communication with the outside world. In other words, the area where polio has reappeared is in the midst of conditions in which access to health care for speedy diagnosis, and also free communication with public health authorities, are likely to be impeded.

At the macro level: Most of the increased incidence in polio infection now is not in the four endemic countries, but in other countries which those four are causing to be reinfected. This has happened repeatedly in Africa; worldwide, of the 333 polio cases so far this year, 222 have been in areas where polio transmission had been interrupted but that then were reinfected. (The WHO’s current tally is here, but they do not use static links, so at some point the numbers behind that link will change.) Just about a month ago, an independent monitoring board said that reinfections of formerly cleared areas are one of the most pressing challenges facing the 23-year eradication initiative.

China is a formerly cleared area: Its last indigenous case was in 1994, and its very last case — imported that time from India — was in 1999. That importation was quelled by wholesale vaccination. The Chinese government has announced more of the same this time: Between now and the middle of September, it plans to immunize 4.5 million children in Hotan and neighboring prefectures. But if the borders of western China are as porous as these four cases suggest, then China and the international eradication effort will have a substantial and dismaying new challenge on their hands.